There is a need for anisotropic optical films that demonstrate negative optical retardation dispersion. For example, a quarter wave film made with negative dispersion birefringent materials will be largely achromatic. Devices such as reflective LCDs that utilise such a quarter wave film will have a dark state that is not coloured. Currently such devices have to use two retarder films to achieve this effect.
The dispersive power of such a negative dispersion birefringent film can be defined in many ways, however one common way is to measure the optical retardation at 450 nm and divide this by the optical retardation measured at 550 nm (R450/R550). If the on-axis retardation of a negative retardation dispersion film at 550 nm is 137.5 nm and the R450/R550 value is 0.82, then such a film will be a largely a quarter wave for all wavelengths of visible light and a liquid crystal display device (LCD) using this film as, for example, a circular polarizer would have a substantially black appearance. On the other hand, a film made with an on axis of 137.5 nm which had normal positive dispersion (typically R450/R550=1.13) would only be a quarter wave for one wavelength (550 nm), and an LCD device using this film as, for example, a circular polarizer would have a purple appearance. Another way of representing this information is to plot the change in birefringence as a function of wavelength. FIG. 1 shows a typical birefringence against wavelength plot for a polymerized film made from the commercially available reactive mesogen RM257 (Merck KgaA, Darmstadt, Germany). The R450/R550 for this compound is around 1.115.
In an anisotropic optical film formed by rod-shaped, optically anisotropic molecules, the origin of the retardation dispersion is due to the fact that the two refractive indices ne, no, of the anisotropic molecules (wherein ne is the “extraordinary refractive index” in the direction parallel to the long molecular axis, and no is the “ordinary refractive index” in the directions perpendicular to the long molecular axis) are changing with wavelength at different rates, with ne changing more rapidly than no towards the blue end of the visible wavelength spectrum. One way of preparing material with low or negative retardation dispersion is to design molecules with increased no dispersion and decreased ne dispersion. This is schematically shown in FIG. 2. Such an approach has been demonstrated in prior art to give LC's with negative birefringence and positive dispersion as well as compounds with positive birefringence and negative dispersion.
If the compounds are polymerizable, or are mixed with a polymerizable host material comprising for example polymerizable mesogenic compounds (also known as “reactive mesogens” or “RMs”), it is possible to prepare anisotropic optical polymer films with negative dispersion. This can easily be carried out by in situ polymerization, e.g. by exposure to heat or UV radiation, of the polymerizable material when being uniformly oriented in its mesophase, thereby permanently fixing the macroscopically uniform orientation. Suitable polymerization methods are well-known to the person skilled in the art, and are described in the literature.
Molecules that can be formed into anisotropic films that demonstrate the property of negative or reverse retardation dispersion have been disclosed in prior art. For example, JP 2005-208146 A1 and WO 2006/052001 A1 disclose polymerisable materials largely based on compounds with a “cardo” core group.
Another class of compounds which is claimed to demonstrate negative birefringence is described in U.S. Pat. No. 6,139,771. These compounds generally consist of two rod-shaped mesogenic groups connected by an acetylenic or bis-acetylenic bridging group. The bridging group is connected to the two rod-shaped groups using a benzene ring on the rod-shaped part of the molecule, resulting in the bridge having an angle of approximately 60° to the rods.
U.S. Pat. No. 6,203,724 discloses molecules generally consisting of two rod-shaped mesogenic groups connected by a highly dispersive bridging group. The bridge is connected to the rod-shaped groups via a the axial position of a cyclohexane ring.
WO 2005/085222 A1 and WO 2006/137599 A1 disclose molecules that have two lower refractive index parts connected by a higher refractive index bridge part. The bridge is predominantly connected to the rods via a fused five-membered heterocyclic ring.
In the compounds described in the above-mentioned documents, where the rods are connected to a highly polarisable bridge, only a maximum of two rods is connected to the polarisable bridging group.
However, many of the materials disclosed in the literature have drawbacks, like for example thermal properties that are not suitable for processing under standard industrial processes, limited solubility in the solvents commonly used in standard industrial processes, or are not compatible with host RM materials commonly used in standard industrial processes, or are too expensive to manufacture.
JP 2005-208414 A1 discloses molecules comprising rod-shaped groups that are covalently bonded to a central discotic group. However in the compounds specifically disclosed in that document, the rod-shaped molecules are connected via an ester group to the discotic ring. The presence of such an ester group gives the rod freedom to rotate in at least two directions relative to the ring. This decreases the probability that the rods and disc will, on average, retain the correct orientation relative to each other, and thus prevents that the desired optical effect can be maximised.
This invention has the aim of providing improved compounds for use in LC formulations and polymer films having negative dispersion, which do not have the drawbacks of the prior art materials.
In particular, there is a need for compounds that demonstrate reduced or negative dispersion, and are also available at reduced cost and with improved properties such as solubility and thermal properties.
Another aim of the invention is to extend the pool of materials and polymer films having negative dispersion that are available to the expert. Other aims are immediately evident to the expert from the following description.
It has been found that these aims can be achieved by providing compounds, materials and films as claimed in the present invention.